Pump



F. E. NORLIN April 4, 1961 PUMP Filed June 6, 1957 kNN PUMP V Francis E. Norlin, Chesterland, Ohio, assigner to Borg- Warner Corporation, Chicago, Ill., a corporation of Illinois Filed June 6, 1957, ser. N6. 664,023

Claims. (Cl. 1113-37) This invention relates to pumps in which a plurality of pump cylinders are arranged in circular series parallel to each other and to the axis of the drive shaft, each of the cylinders having disposed therein a piston or plunger actuated by a cam or swash plate fixed on the shaft.

The pump forming the subject matter of the invention is provided with means to vary the displacement thereof, such means including means responsive to the discharge pressure from the cylinders, and the pump is further provided with means allowing a selection of a particular maximum discharge pressure therefrom. Such means are desirable in pumps Iwhich are constantly running, as forl claimed in copending applications, Serial No. 573,543,

filed March 23, 1956, entitled Variable Displacement Hydraulic Pump and Serial No. 598,718, filed luly 18, 1956, now Patent No. 2,930,321, entitled Pump Therefore, the primary object ofthis invention is the provision of an improved variable displacement pump of the type incorporating a plurality of circularly arranged cylinders having pistons therein actuated by means of a cam or swash plate. p

More particularly, the pump of this invention includes pistons, each having an open-ended cavity therein in which is disposed a valve means adapted to seat and close the open end of the piston cavity, together with means for varying the position of closing the open end of the piston cavity by means of the valve during the piston discharge stroke to thereby effectively vary the effective discharge stroke of the piston and the displacement of the pump, the latter means including a pilot valve responsive to pump discharge pressure. ln the aforementioned copending application Serial No. 573,543 the means for varying the effective discharge stroke of the pistonsrincludes a spider or a plate provided with a plurality of radially extending fingers movable by means of a spider piston responsive to a pressure related to the pump discharge pressure, so as to move the spider piston and, therefore, the spider between limits at whichothe pump will provide full and zero displacement, respectively. ln the copending application, Serial No. 598,718, the spider or plate is replaced with means comprising a plurality of separate, individual, radially extending fingers, one for each piston, movable in response to a pressure related to the pump discharge pressure applied to the supporting stems of each finger.

The means for varying the effective discharge strokes of the pistons, forming the subject matter of the instant invention includes the same type spider or plate, together with the spider piston as set forth in the copending application, Serial No. 573,543, and, in addition is provided with means in the form of valves for allowing a relief of the pressure acting on the spider piston should thel load on any one of the spider fingers become excessive, as will ,be later explained. Also, the spider is mounted so i y United States Patem;1

, 2,977,887 Patented Apr. 4, 1961 'as to be capable of a slight tilting movement. Therefore, another object of this invention is the provision of an improved pump including a plurality of parallel cylinders, each having a piston reciprocated therein by means of a cam or swash plate, so as to each have. a suction stroke and a discharge stroke, valve means associated with each of the pistons and adjustable, so as to vary the effective discharge stroke of the piston to vary the displacement of the pump, spider means to provide the adjustment, and means to relieve the pressure acting on the spider means upon an excessive load thereon.

More particularly, a still further object of this invention is the provision of an improved variable displacement piston swash type pump including means comprising a spider having a plurality of radially extending fingers and responsive to a fiuid pressure related to the discharge pressure of the pump to vary the displacement thereof, and valve means to relieve the related pressure upon an excessive load being impressed on the fingers.

These and other objects andfeatures of this invention will be apparent from the following specification when taken with the accompanying drawing, in which:

Figure 1 is a partial sectional view of a ptunp constructed in accordance with the invention;

Figure 2 is an enlarged sectional View of the control means of the pump of Figure l; and

Figure 3 is an enlarged partial sectional View of the valve means to relieve the pressure upon an excessive load on the pump spider fingers.

Referring now to the drawings,'there is illustrated a pump, generally indicated at 10, having a housing comprising a body 11 and a cover 12. The cover 12 has an inlet connection 13 and an outlet connection 14 which take the form of tapped ports into which the inlet and discharge pipes or hoses (not shown) are screwed. The body 11 and cover 12 are maintained in their illustrated abutting position by means such as bolts or the like (not shown).

A cylinder assembly is clamped between a shoulder 15 in the body 11 and a shoulder 16 in the cover 12, and the cylinder assembly is made up of a cylinder barrel block 17 and a guide block 18. The blocks 17 and 18 are annular in form and define with the cover 12 an inlet chamber 19 which communicates with the inlet port 13 by way of a passage 20 formed in the cover 12. The cylinder block 17 defines with the cover 12 a discharge chamber 21 which communicates with the outlet or discharge port 14 by way of a passage 22 formed in the ,cover 12.

The guide block 18 is restrained from any tendency to rotate by means of a retainer element 23 having an enlarged portion 24 threadably received in an internally threaded boss-like portion 25 of the body 11 and extending within a cavity 26 in the block 18. The retainer member 23'is provided with a central bore 27 in alignment with a bore 28 in the block member 18. The function of the bores 27 and 28 will be later described. However, it may be seen that the internally screw-threaded portion 25 of the body 11 may receive a suitably threaded hose or pipe. The cylinder barrel block 17V is bored through to form a plurality of parallel arranged cylinders 29, and the Vblock 17 is formed to define with the block 18 inlet passages 30 which communicate with the cylinders 29. While only one cylinder is illustrated in the drawings,`the usual number of cylinders is nine (9). The guide block 1 8 is bored through at 31, these guide bores 31 being of the same diameter and in alignment with the cylinders 29; A discharge valve 32 associated with each cylinder 29 `is seated by a coiled compression spring ,33 against ,the outer face of the cylinder barrel block 17. The discharge valves 32 are guided in .pockets 34 formed in the cover 12, and

are disposed Within the discharge chamber 21, as illustrated.

A piston or plunger 35 is slidably received in each of the cylinders 29 and extends through a guide bore 31 and the block member 18, terminating in an enlarged shoulder portion 36 located in a cavity 37 Within the body 11 and behind the block 18. Each piston 35 is provided with a cavity 38 open at the end thereof adjacent the associated discharge valve 32, a cylindrically elongated inlet port 39 in communication with the inlet passage 30 when the piston is in the position illustrated in Figure l, and an elongated slot 40 for a purpose to be described.

A swash or cam plate 41 disposed within the cavity 37 is provided for reciprocating the pistons or plungers 35 between the position shown and a position in which the pistons are fully displaced within their cylinders 29, which positions correspond, respectively, to full suction and full discharge positions. The cam plate 41 is provided with a hub 42, journalled in suitable needle bearings 43, to which is connected, by axial thrust means 44, a splined driving shaft 45. The thrust means 44 being conventional needs no description.

Each piston 35 is provided with a substantially hemispherical cavity 46 in the enlarged portion 36 to receive a complementary-shaped bearing shoe 47 having a flat surface which bears against the face of the cam plate 41. The needle bearings 43 are retained in an annular retainer member 43 fixedly positioned in the body 11, which is also provided with hemispherical cavities 49 to receive complementary-shaped bearing shoes 50 having flat surfaces to engage the opposite side of the cam plate 41 than the bearing shoes 47. The pistons are retracted by means of a piston return plate 51 having fork-like portions 52 engageable with the enlarged portions 36 of the pistons. The piston return plate is provided with a centrally located hemispherical cavity 43 which is disposed over a complementary-shaped end portion 54 of a pilot valve retainer member 55, the purpose of which will be later explained, received in a central bore 56 in the guide block 18. The portion 54 of the pilot valve retainer member 55 maintains the piston return plate 51 in its operating position.

A valve member 57 having a frusto-conical portion 58 adapted to seat on a correspondingly shaped surface portion 59 of the piston 35 is provided for closing the open end of each piston 35 during at least a portion of the pressure or discharge stroke of the piston. The valve member 57 is provided with an axially extending stem member 60 having an enlarged end portion 61 and is slidably supported within the piston 35 by means of a support member 62 fixedly disposed within the cavity 38 of each piston 35. A coil compression spring 63 surrounds the stem member 60 and is disposed between the enlarged end portion 61 and the support member 62 to resiliently urge the valve member to its illustrated position, wherein the frusto-conical portion 58 is seated on the surface 59. As will be obvious by varying the position, relative to the cylinder 29, in which the valve 57 closes the open end of a piston, as by seating on the surface 59, the effective discharge stroke of the piston 35 will be varied, and such variation Will vary the displacement of the pump.

In order to vary the position at which a valve 57 seats on a surface 59 of a piston 35, means in the form of a spider 64 having a plurality of radially extending fingerlike elements 65, one for each piston, is provided, the finger-like elements extending through the elongated slots in the pistons 35 to a position wherein they may each contact the enlarged valve stem portions 61. The spider 64 is axially movable between certain limits corresponding to full and zero pump displacement and is supported for such axial movement on a cylindrical extension 66 of an annular spider piston 67 disposed within a cavity 68 inthe guide block 18, and defining with the cavity 68 a chamber 69, hereinafter referred to as the spider piston chamber. It should be noted here that the passage 28, previously described, communicates with the spider piston chamber 69, as illustrated. The fit of the spider 64 with the supporting cylindrical extension 66 is such that the spider may rock slightly thereon. An annular bearing member 70 is disposed around the cylindrical portion 66 and abuts the spider 64. The bearing member 70 is provided with a plurality of spaced apertures 71 having conical walls forming seats for an equal number of balls 72 seated also in the mouths of openings 73 formed in the spider piston 67 (see Figure 3). As will be obvious, when the spider 64 moves in a purely axial manner, the balls act to prevent communication between the inlet cavity 19 and the chamber 69, but if the spider becomes cocked, as will be later explained, at least one of the balls will become unseated and allow such communication.

A cylindrical flanged retainer member 74 is slidably received around a cylindrical extension 75 of the spider piston 67, the diameter of the extension 75 being less than the diameter of the extension 66, so that sliding movement of the retainer member 74 is limited by a shoulder 76 formed at the juncture of the extensions 66 and 75 in one direction. Therefore, it may be seen that the axial movement of the spider 64 is limited by the member 74 and the member 70. A coil compression spring 77, hereinafter referred to as a spider return spring is received between the retainer member 74 and a closure member 7S received in an opening 79 in the cover 12. The closure member 78 is provided With a reduced cylindrical extension portion 80 which receives and supports the reduced cylindrical portion 75 of the spider piston 67 in telescopic arrangement, as illustrated. The spider 64 is guided in its axial movement by means of spaced fingers 81 extending from and integral with the guide block 18.

The pilot valve retainer 55, received in the bore 56, as is obvious from the drawing, also includes a portion received Within the annular piston 67. The retainer member 55 is provided with open-ended concentric communicating bores 82 and 83 (see Figure 2), the bore 83 being of a smaller diameter than the bore 82, and is transversely drilled as at 84 to form a communication between the outer periphery thereof and the bore 83. The retainer member 55 is also provided with a reduced annular groove 85 intermediate its ends and which defines with the piston 67 and the bore 56 a chamber 86, for a purpose to be described. It should be noted that the chamber 86 communicates with the piston chamber 69. The retainer member 55 is also provided with transverse bores or openings 87 and 88 providing communication between the bore 82 and the chamber 86.

A pilot valve 89 is slidably received within the bore 82 and is provided with an open-ended cavity 90, a first reduced portion 91, a groove 92, a reduced portion 93, and a reduced cylindrical portion 94 defining a shoulder 95 with the portion 93. A pilot valve sleeve 96 is disposed within the bore S2 and is fixedly retained in position by means of a fastening element 97. The sliding movement of the pilot valve 89 is limited by the shoulder 95 engaging the sleeve 96 in one direction and the other end of the pilot valve 89 engaging the shoulder formed by the intersection of the bores 82 and 83y in the other direction. The pilot valve 89 is transversely bored, as at 98, to provide communication between the groove 92 and the bore 90, and the reduced portions 93 and 94 and the sleeve 97 define with the bore 82 a pilot ring pressure chamber 99.

A pin member 100 is disposed within the cavity 90 at one end and within a cavity 101 in a cup-like spring retainer member 102 at its other end. The spring retainer member 102 is elongated and has an annular portion slidably engageable with the interior of the cylindrical member 75 extending from the piston 67. A coil compression spring 103, hereinafter referred to as a pilot valve spring,

is disposed within the cup-like spring retainer member 102 at one end and at its other end is-supportedby means of a tip portion 104 extending from a plug portion 105 of an adjusting screw-106 threadably received'in the member 78, as illustrated. The adjusting screw 106 extends to a position outside the cover 12 and is provided with a slot 107 to receive means, such as a screw driver, toadjust its position and thereby adjust the loading of the spring 103. The spring 103, as is obvious, resiliently urges vthe pilot valve 89'to its illustrated position (see Figure 2) through the pin member 100. An acorn nut 108 is threadably received over the screw threaded adjusting screw 106'to prevent damage thereto.

Openings 109, 110 and 111 are provided in the portion 75 and in the retainer 102 for the passage of uid therethrough, this being necessary and apparent from the operation of the pump,l later to be described.

The guide block 18 is provided with an annularV chamber 112 in communication with the openings 84, and is also provided with an angularly disposed passage 113` in communication at one end with the chamber 112 and at the other end with a passage 114 formed in the cylinder barrel block 17. The passage 114 in the block 17 conrmunicates with the discharge chamber 21 for the passage of discharge pressure to the chamber 112.

An air lock release valve, generally indicated at 115 is provided, but since this does not form a part of the inventionherein described, further and detailed description is unnecessary, except to say that the valve 115 is identical to that describedand claimedin the aforementioned copending application, Serial No. 573,543, to which reference is made.

Suitable O-ring sealing means are provided where necessary as is usually the practice in devices of this nature.

The following description sets forth the operation of the pump and this operation will be described with reference to one of the plurality of pistons, it being understood that the operation of all pistons is the same.

Assuming the shaft 45 is rotating, it will be apparent that the cam or swash'plate 41 will be rotating and the pistons 35 will be reciprocated Within the cylinders 31 between their suction positions and their discharge positions. Fluid from asuitable source will ow to the inlet 13, into. the passage 20 and into the chamber 19 filling the entire chamber. From the chamber 19 the liuid will ow through the passage 30, and when the piston is in its suction position, through the openings 39 and 40 and into the cavity 38 of the piston 35. The uid Within the cavity 38 exerts a force on the piston valve 57 causing it to unseat and move away from the end of the piston, so that fluid will ow around the valve 57 and into the cylinder 29 in advance of the piston, the liuid being retained therein by means vof the seated discharge valve 32. As is obvious, on the discharge stroke of the piston 35 the fluid Within the cylinderv29 will be placed under pressure causing the valve 57 to close.

As the piston moves to the left on its discharge stroke, compressing the fluid trapped within the cylinder 29, the discharge valve will be unseated from the end of the block 17, so that the fluid under pressure will ow into the discharge chamber 21, through the discharge passage 22 and out the outlet 14. At the same time, high pressure fluid from the discharge chamber 21 will flow through the passage 114, through the passage 113, and into the chamber 112. The fluid in the chamber 112 will ilow through the openings 84 and into the bore 83 where it contacts one end of the spool-type pilot valve 89.

When the pressure in the bore 83 becomes suiciently high, or, in other words, the discharge pressure becomes sufficiently high, so that the force on the end of the pilot valve 89 overbalances the spring 103, the pilot valve will moveto Vthe left as illustrated in the drawing, and compresses the pilot valve spring 103. The pilot valve discharge pressure.

Y discharge pressure will allow the flow of lluid around the spring -103 may be` adjusted by means of the member 106 to Va'load which will allow the pilot valve 89 to reach a position providing a bleed between the reduced portion 91 and the openings 88at a predetermined outlet or Atthis point a slight increase in reduced portion 91 to bleed into the piston chamber `69 through the openings 88. At this time it will be noted that the groove 92 is blocked by the interior ofthe bore 82. Pressure will build up in the piston chamber 69 suflicient to overcome the return spring 77, and the duid thus acting on the piston 67 will start moving the spider 64 to the left, as viewed inthe drawing. Fluid will also ow into the openings 73 from the chamber 69, vbeing retained therein by means of the balls 72.

After the spider 64 has moved a short distance, a iinger portion 65 thereof starts to contact the enlarged end 610i a piston valve 57 and forces the valve to open or to vremain open for part of the pressure or discharge stroke. This allows fluid in the piston cylinder 29 to bypass back to inlet and also prevents pressure in the piston cylinder 29 from building up until the piston has reached a point in its travel toward its discharge position where it picks up the valve 57 and pulls the valve away from` the finger portion 65.

A slight decrease in outlet -or discharge pressure will allow the pilot valve 89 to move toward its position illustrated and thereby shut ol the outlet tiow to the chamber 69 and will allow tiuid trapped in the cham-ber 69 to bleed out through the passage 88, into the groove 92, through the passage 98, through the bore and to the chamber 19 through the openings 110, 111 and 109. The spider return spring 77 will then start returning the spider 64 to its illustrated position, which will allow the piston valve 57 to close off sooner, thereby increasing the effective piston stroke which in turn increases the pump displacement.

A steady, partial flow demand will cause the pilot valve 89 to assume a position in which pilot valve leakage or bleed into the spider piston chamber 69 will equal pilot valve leakage out of the piston chamber 69 back to the inlet cavity 19 through the passage 88, the groove 92, the passage 98 and the bore 90 thus holding the spider 64 in a constant partial flow position. Since the pilot valve 89 travels only a few thousandths of an'inch in the bore 82 to bleed ow into and out of the spider chamber 69, the pilot valve spring vload increase due to this travel is very slight. Also, since the pilot valve 89 assumes the same position for any partial ow demand, the load on the spring 103 is the same, and therefore, the outlet pressure will be the same. Thus, there would be no pressure differential between maximum pressure at full ow and at zero how. But, there must be a pressure change in order to controlV the location of the spider 64, and this pressure change, also called the pressure differential, has been built into the pilot valve 89. Thus, a cut-off slope or angle is provided on a pressure versus dow curve, which corresponds to the pressure differential.

To provide the pressure differential is the function of the pilot ring pressure chamber 99. As will be obvious, when fluid ilows to the chamber 69, there will also be a flow to the chamber 86, through the opening 87 and into the chamber 99. When this chamber is under pressure, it balances out part of the outlet pressure which contacts the end of the pilot valve 89. The pressure in the chamber 99 increases as the spider 65 moves toward zero displacement position (the limit of the spider movement to the left, as illustrated) due to the increas- 1 ng spider return spring load. As the pressure increases 1n the pilot ring pressure chamber 99, the outlet pressure of the pump must `increase to overcome this pressure and this creates the pressurediferential between full and zero ow.

Thus, the pilot ring pressure chamber 99 hydraulically applies the spring rate of the spider return spring 77 to the pilot valve 89. During partial displacements of the pump, the spider return spring load varies with the position of the spider 64. As the spider return spring load varies the pressure in the spider piston chamber 69 and in the pilot ring chamber 99 varies. Since the pilot ring chamber 99 applies this pressure to the pilot valve 89 against outlet or discharge pressure, the outlet pressure must vary to maintain the pilot valve 89 in balance. Therefore, a definite link has been established between the location of the spider 64 and outlet pressure, and since the location of the spider 64 establishes the correct displacement, it can be said that outlet pressure establishes the correct displacement.

Without this pilot ring chamber 99, the valve S9 would crack at a set pressure and close off at a set pressure regardless of the location of the spider 64. This is so because the spider 64 can move from the full displacement position (as illustrated) to the zero displacement position (the limit of its movement to the left) without increasing the load of the pilot valve spring 103. Therefore, there would be no change in outlet pressure whether the pump be in, for example, 75% displacement, 50% displacement, or Zero displacement. If there were no change in outlet pressure there would be no means of stopping the spider 64 at any desired location, and as a result, the spider would hunt back and forth, resulting in undesirable chatter.

Zero flow mayalso be provided by connecting a low pressure source of fluid to the passages 27 and 28, so that it will act on the piston 67 and move the spider to its zero displacement position.

If in the movement toward the left the spider lingers 65 move too rapidly as when there is a surge of pressure, or there is a great resistance to movement of the piston valves 57, the fingers will contact the spider valves with great force, possibly great enough to crack them from the spider. However, by the construction herein set forth, the spider will cock and allow one of the balls J72 to become unseated, thereby relieving the pressure in the spider piston chamber by providing communication with the inlet cavity 19 and allowing the spider to move with the force applied thereto, preventing the cracking of the fingers.

While this invention has been described in connection with a certain embodiment thereof, it is to be understood that this is by way of illustration and not by way of limitation and the scope of the invention is dened solely by the appended claims which should be construed as broadly as the prior art will permit.

I claim:

l. In a pump of the type having a plurality of circularly arranged parallel pistons reciprocated within cylindrical bores by means of a cam plate so as to each have a suction `and a discharge stroke, the improvement comprising, an open-ended cavity in each piston, means to vary the effective discharge stroke of each piston, said means comprising a valve carried by each piston and adapted to seat on the open end thereof to close said open end, each valve including a stem portion, means to resiliently urge each valve to its seating position, means adapted to contact each stem portion to thereby unseat each valve during at least a portion of its discharge stroke, said last-named means being interconnected with and being movable by a. fluid motor, means connecting said fluid motor to a source of fluid pressure so as to be responsive thereto whereby said contacting means is moved and varies the etective stroke of each piston in response to said fluid pressure, and means comprising at least one ball check valve in communication with said tluid pressure to relieve said uid pressure when the load on said contacting means as it contacts said stem portion becomes excessive.

2. In a pump of the type having a plurality of circularly arranged parallel pistons reciprocated within cylindrical bores by means of a cam plate so as to each have a suction and a discharge stroke, the improvement wmprising, lan open ended cavity in each piston, means to vary the effective discharge stroke orf each piston, said means comp-rising a valve carried by each piston and adapted to seat on the open end thereof to close said open end, each valve including a stem portion, means to resiliently urge each valve to its seating position, means adapted to contact each stem portion to thereby unseat each valve during at least a portion of its discharge stroke, said last-named means being interconnected with and being movable by -a fluid motor, means connecting said uid motor to a source of fluid pressure so as to be responsive thereto whereby said contacting means is moved and varies the effective stroke of each piston in response to said fluid pressure, and means to relieve said fluid pressure when the load on said contacting means as it contacts said stem portion becomes excessive comprising a plurality of circularly arranged ball check valves, and a plurality of passages in said fluid motor, said passages being in communication with said uid pressure, said ball check valves being adapted to block said passages when said load is normal.

3. In a pump of the type having a plurality of circularly arranged parallel pistons reciprocated within cylindrical bores by means of a cam plate so as to each have a suction stroke and a discharge stroke, the improvement comprising an open-ended cavity in each piston, a valve carried by each piston adapted to seat on the open end thereof and effective to close the open end, each valve including a stem portion, means to resiliently urge each valve to its seating position, means movable between positions corresponding to full pump displacement and zero pump displacement comprising a spider having a plurality of finger-like elements each of which is individual to a piston and adapted to each contact a valve stem portion during at least a portion of the discharge stroke so as to unseat the valve, resilient means urging said movable means to its full displacement position, a uid motor having a piston and being interconnected with said movable means and movable between positions corresponding to the positions of said movable means, means providing a pressure related to pump discharge pressure, means communicating said related pressure to said uid motor so as to move said piston and said movable means towards said zero pump displacement position, said movable means and said valve thereby varying the effective discharge stroke of said piston to vary the displacement of said pump in response to said related pressure, and at least one ball check valve in uid communication with said related pressure to relieve the related pressure when the load on a finger-like element becomes excessive as it contacts said stem portion.

4. In a pump of the type having a plurality of circularly arranged parallel pistons reciprocated within cylindrical bores by means of a cam plate so as to each have a suction stroke and a discharge stroke, the improvement comprising an open-ended cavity in each piston, a valve carried by each piston adapted to seat on the open end thereof and effective to close the open end, each valve including a stem portion, means to resiliently urge each valve to its seating position, means movable between positions corresponding to full pump displacement and zero pump displacement comprising a spider having a plurality of finger-like elements each of which is individual to a piston and adapted to each contact a valve stern portion during at least `a portion of the discharge stroke so as to unseat the valve, resilient means urging `said movable means to its full displacement position, a

discharge stroke of said piston to vary the displacement.

of said pump in response to said related pressure, a plurality of circularly arranged ball check valves, means Y defining a plurality .of passages through said piston pressure when the load on a finger-like element becomes v excessive as it contacts said stem portion.

5. In a pump of the type having a plurality of circularly arranged parallel pistons reciprocated within cylindrical bores by means of a cam plate so as to each have a suction stroke and a discharge stroke, the improvement comprising an open-ended cavity in each piston, a, valve carried by'each piston adapted to seat on the open end thereof and effective to close the open end, each valve including a stem portion, means to resiliently urge each valve to its seating position, means movable between positions corresponding to full pump displacement and zero pump displacement comprising a spider having a plurality of linger-like elements each of which is individal to a piston and adapted to each contact a valve stem portion during at least a portion of the discharge stroke so as to unseat the valve, resilient means urging said movable means t-o its full displacement position, said spider further being mounted for cooking motion, a uid Ymotor having a piston and being interconnected with said movable means and movable between positions corresponding to the positions of said movable means, means providing a pressure related to pump discharge pressure,

means communicating said related pressure to said iluid motor so as to move said piston and said movable means towards said Vzero pump displacement position, said movable means and said valve thereby varying the effective discharge stroke of said piston to vary the displacement l of said pump in response to said related pressure, and

means to relieve the communicated pressure when the load on a finger-like element becomes excessive and said spider cocks as it contacts said stern portion.

References Cited in the iile of this patent UNITED STATES PATENTS 2,848,954 Budzich Aug. 26, 1958 

